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The Formation of a Cooling-induced Mesovortex in the Trailing Stratiform Region of a Midlatitude Squall Line

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Abstract

The structure and evolution of a cooling-induced midlevel mesovortex, referred to as the wake vortex, which developed in the descending rear inflow of the 10-11 June 1985 squall line has been studied, principally based upon a 20-h real-data simulation of the case. The model simulation is verified against all available observations in prior studies. A 3D vorticity budget indicates that the cooling-induced vortex is initially maintained through the vertical stretching of its absolute vorticity associated with the short-wave trough. 54 refs.
... As mentioned above, previous studies have proposed several mechanisms that can lead to the formation of an SWV and, among them, from case to case, some factors are similar while others show distinct differences. This is similar to the situation in the mesoscale convective vortex (MCV) research (Zhang, 1992), that is, different cases show different features. This means that case studies may be limited to reflect the universal features of the mechanisms underlying the SWV formation. ...
... Similar to previous studies (Zhang, 1992;Olsson and Cotton, 1997;Sun et al., 2010;Fu et al., 2017), the vorticity budget is used here to understand the mechanisms of SWV formation. The equation documented in Kirk (2003) is as follows: where ζ is relative vorticity, V h = ui + vj is the horizontal wind vector, f is the Coriolis parameter, β = ∂f/∂y, r h = ∂ ∂x i + ∂ ∂y j is the horizontal gradient operator, p is pressure, and ω = dp/dt. ...
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On the basis of a 14‐year statistical dataset of the southwest vortex (SWV), 10 long‐lived and heavy rain–producing SWVs are selected for a series of convection‐permitting composite semi‐idealized simulations. After confirming the simulation has reproduced the common salient features of these selected vortices, the general underlying mechanisms for the formation of this type of SWV are investigated. Results show that the SWV formation is characterized by significant unevenness, as the fundamental characteristics and dominant factors for the cyclonic‐vorticity production vary from place to place. Overall, the eastern/western section of SWV mainly features favorable/unfavorable conditions for the cyclonic‐vorticity enhancement. The vorticity budget indicates the convergence‐related horizontal shrinking and convection‐related tilting contribute to the cyclonic‐vorticity enhancement in the SWV's eastern section, whereas in the western section, the terrain‐related tilting and horizontal transport result in the cyclonic‐vorticity reduction. On the basis of a 14‐year statistical dataset, the universal mechanisms underlying the formation of a typical type of heavy rain–producing of southwest vortices (SWVs) is derived as the above shows (VAV = vertical advection of vorticity; STR = stretching; TIL = tilting. A bigger size of a sign means larger intensity). The SWV formation features significant unevenness, the eastern/western section of the SWV has favorable/unfavorable conditions for cyclonic‐vorticity enhancement. The horizontal convergence‐related shrinking and convection‐related tilting favor cyclonic‐vorticity enhancement in the SWV's eastern section, whereas in the western section, terrain‐related tilting and horizontal transport cause cyclonic‐vorticity reduction.
... There have been several numerical modelling studies that focus on the development of MCVs at mid-latitudes (Zhang, 1992;Chen and Frank, 1993;Rogers and Fritsch, 2001;Conzemius and Montgomery, 2009;Davis and Galarneau, 2009;Wang et al., 2013). These studies have not emphasized a particularly strong role of sublimation in causing mid-level vortices to form, with the exception of Zhang (1992) who found that cooling in the trailing stratiform region of a squall line appeared to play a dominant role in causing a mesoscale vortex to form and sublimation as well as melting and evaporation being responsible for the cooling. ...
... There have been several numerical modelling studies that focus on the development of MCVs at mid-latitudes (Zhang, 1992;Chen and Frank, 1993;Rogers and Fritsch, 2001;Conzemius and Montgomery, 2009;Davis and Galarneau, 2009;Wang et al., 2013). These studies have not emphasized a particularly strong role of sublimation in causing mid-level vortices to form, with the exception of Zhang (1992) who found that cooling in the trailing stratiform region of a squall line appeared to play a dominant role in causing a mesoscale vortex to form and sublimation as well as melting and evaporation being responsible for the cooling. Also, some modelling studies of mid-latitude squall lines indicate that sublimation cooling occurs in the rear inflow air as it begins to descend under the cloud deck (Yang and Houze, 1995;Braun and Houze, 1997). ...
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p>Mid-tropospheric mesoscale convective vortices have been often observed to precede tropical cyclogenesis. Moreover, recent cloud-resolving numerical modelling studies that are initialized with a weak cyclonic mid-tropospheric vortex sometimes show a considerable intensification of the mid-level circulation prior to the development of the strong cyclonic surface winds that characterize tropical cyclogenesis. The objective of this two-part study is to determine the processes that lead to the development of a prominent mid-level vortex during a simulation of the transformation of a tropical disturbance into a tropical depression, in particular the role of diabatic heating and cooling. For simplicity simulations are initialized from a quiescent environment. In this first part, results of the numerical simulation are described and the response to stratiform components of the diabatic forcing is investigated. In the second part, the contribution of diabatic heating in convective cells to the development of the mid-level vortex is examined. Results show that after a period of intense convective activity, merging of anvils from numerous cells creates an expansive stratiform ice region in the upper troposphere, and at its base a mid-level inflow starts to develop. Subsequently conservation of angular momentum leads to strengthening of the mid-level circulation. A 12 h period of mid-level vortex intensification is examined during which the mid-level tangential winds become stronger than those at the surface. The main method employed to determine the role of diabatic forcing in causing the mid-level inflow is to diagnose it from the full physics simulation and then impose it in a simulation with hydrometeors removed and the microphysics scheme turned off. Removal of hydrometeors is achieved primarily through artificially increasing their fall speeds 3 h prior to the 12 h period. This results in a state that is in approximate gradient wind balance, with only a weak secondary circulation. Then, estimates of various components of the diabatic forcing are imposed as source terms in the thermodynamic equation in order to examine the circulations that they independently induce. Sublimation cooling at the base of the stratiform ice region is shown to be the main factor responsible for causing the strong mid-level vortex to develop, with smaller contributions from stratiform heating aloft and low-level melting and evaporation. This contrasts with the findings of previous studies of mid-latitude vortices that indicate sublimation plays a relatively minor role. An unanticipated result is that the central cool region that develops near the melting level is to a large degree due to compensating adiabatic ascent in response to descent driven by diabatic cooling adjacent to the central region, rather than in situ diabatic cooling. The mid-level inflow estimated from stratiform processes is notably weaker than for the full physics simulation, suggesting a moderate contribution from diabatic forcing in convective cells. .
... As previous studies (Zhang, 1992;Fu et al., 2013Fu et al., , 2017Feng et al., 2019; show, vorticity budget is effective to determine the key mechanisms underlying vortices' variation. In this study, we calculated the area (within the central region of the vortex) averaged vorticity budget, which can represent the variation of the velocity circulation of the vortex (i.e., the vortex's intensity) effectively according to the Green's theorem (Fu et al., 2017;Fu et al., 2019). ...
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From 17 to 22 July 2021, a series of disastrous rainstorms appeared within Henan Province and its surroundings, which rendered 398 dead/missing and a direct economic loss of up to ∼114 billion Yuan. This event was named as the “21.7” Henan torrential rainfall event (TRE), and it could be divided roughly into three stages according to the precipitation features and dominant weather systems. This study mainly focused on the development and maintenance mechanisms of a long-lived (72 h) quasi-stationary mesoscale vortex that governed the earlier stage of the “21.7” TRE. Main results are as follows: 1) the mesoscale vortex formed/maintained in a favorable environment which was characterized by the South-Asia-high associated divergence in the upper troposphere, and the relay transport of relatively cold air by the trough and high-pressure system in the middle troposphere. 2) the vertical stretching due to lower-level convergence and the upward transport of cyclonic vorticity by ascending motions served as the first and second dominant factors for the mesoscale vortex’s rapid development and long-time maintenance, whereas, the import/export transport of anticyclonic/cyclonic vorticity into/from the vortex’s central region and the tilting effects were mainly detrimental for the vortex’s development/maintenance. 3) the air particles that formed the mesoscale vortex mainly came from the levels below 1,500 m, within the regions in lower latitudes. The air particles experienced notable precipitation before the formation of the mesoscale vortex, which contributed to the vortex’s formation through latent heat release.
... 2000), the MCV in this article was typical in its diameter, thickness, intensity, and formation time, but longer in life span (i.e., 21 hr). Compared with the various mechanisms that favored MCVs' formation/sustainment (e.g., Clark et al., 2010;Cram et al., 2002;Galarneau et al., 2009;Kirk, 2003Kirk, , 2007Knievel & Johnson, 2003;Sun et al., 2010;D.-L. Zhang, 1992), the most distinctive feature for the MCV in this event was that the horizontal advection acted as a dominant factor, whereas the term STR (i.e., divergence-related vertical shirking) mainly served as a detrimental factor. Differences in the background environment were an important reason for the differences in mechanisms. Another featu ...
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From 17–22 July 2021, Henan Province experienced the most severe torrential rainfall event since 1975 with a maximum hourly precipitation of 201.9 mm appeared in Zhengzhou, which was the largest hourly rainfall thus far observed by meteorological observation stations over the Chinese Mainland. The appearance of a long‐lived (21‐hr) northwestward‐moving mesoscale convective vortex (MCV) and its interaction with its parent mesoscale convective system (MCS) was crucial to produce the extremely strong heavy rainfall in Zhengzhou. The backward trajectory analysis indicates that air particles in the lower troposphere beneath the MCS over Henan contributed mostly to the MCV's formation. These air particles experienced notable ascending motions and condensation with their strong cyclonic vorticity mostly produced 1‐hr before the MCV's formation. Vorticity budget denotes that strong upward transport of cyclonic vorticity and convergence‐related vertical stretching, both of which were mainly due to convection associated with the parent MCS, acted as dominant factors for the MCV's formation. After formation, the MCV first coupled with its parent MCS, during which its intensity, thickness, and precipitation were all maximized; then, it moved northwestward and decoupled from the MCS, during which it weakened rapidly and finally dissipated. Convection‐related upward cyclonic vorticity transport and inward horizontal advection of cyclonic vorticity associated with an inverted trough over the Henan Province dominated the vortex's development/maintenance in the coupling stage; whereas outward horizontal advection of cyclonic vorticity dominated the MCV's dissipation after it completely decoupled from its parent MCS. These differ notably from the findings documented in previous MCV‐related literature.
... The evolution of the SWV can be represented by vorticity effectively (Fu et al., 2010;Feng et al., 2020), the diagnosis of vorticity budget was used here to probe into the detailed evolution process from a single-core SWV to the DCSWV. According to Zhang (1992), the vorticity equation can be written as Eq. 1: ...
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The southwest vortex (SWV), a low-pressure system bringing severe rainfall in southwest China, is one of the most important synoptic systems in China. Using both the National Centers for Environmental Prediction Final (NCEP-FNL) operational global analysis dataset and the Weather Research and Forecasting (WRF) model simulation, a sophisticated SWV with dual-core structure (DCSWV) over the Sichuan Basin in 2010 was studied. The DCSWV system consisted of two cores, one near Leshan City (named “C1”) and another near Langzhong City (named “C2”). The high-resolution WRF model reproduced the life cycle of the DCSWV well. The diagnostic analysis of the vorticity budget indicated that the stretching and tilting terms played important roles in the development stage of “C1”, while the stretching and vertical advection of vorticity were the major contributors to the formation and development stage of “C2”, which implied the importance of moisture convergence and ascending motion. Sensitivity experiments showed that the DCSWV was closely associated with the release in latent heat as well as the effect of topography. The great release in latent heat provided significantly positive feedback to the DCSWV system, which was decisive to the formation and development stages of “C2”. The topography of the Tibetan Plateau and the Yun-Gui Plateau affected the location and duration of the DCSWV.
... Over the years, domestic and foreign meteorologists have studied the organizational structures of MCSs and the interactions between MCSs and large-scale circulations from the following six aspects. (1) The structure and conceptual model of ascending and descending air flows in MCSs (Houze et al. 1989;Zipser 1977;Bryan and Fritsch 2000;Moncrieff and Klinker 1997); (2) The formation mechanism of MCV in the middle level of MCSs (Skamarock et al. 1994;Chen and Frank 1993;Zhang 1992); (3) The organization, movement and propagation of MCSs (Corfidi et al. 1996;Rotunno et al. 1988;Fritsch et al. 1994;Fritsch and Forbes 2001;Doswell III et al. 1996;Carbone et al, 2002); (4) Impact factors of the duration and size (Webster et al. 2002;Chen et al. 1996;Williams and Houze 1987); (5) The feedback of MCSs to largescale environments through the momentum and heat exchange (LeMone 1983;Yuter and Houze 1998.); (6) The global distribution and impact of MCSs (Nesbitt et al. 2000;Schumacher and Houze 2003). ...
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On the morning of July 21, 2017, a localized rainstorm event occurred in the Shijiazhuang area of Hebei Province, with the heavy rain mainly concentrating in the Shijiazhuang urban area and its eastern and northern parts. But this rainstorm process was omitted by both numerical and subjective forecasts. In this paper, the triggering mechanisms of the mesoscale convective systems that have caused this localized rainstorm are analyzed by using the intensified surface observation data, Doppler radar data, the four-dimensional variational Doppler radar analysis system (VDRAS), the National Centers for Environmental Prediction (NCEP) reanalysis data and the ERA5 reanalysis. The observation analysis shows that when the heavy rain was occurring, the Shijiazhuang area was controlled by the western Pacific subtropical high, and the high temperature and high humidity environment has provided favorable vapor, thermal and energy conditions for the rainfall. Three key factors are responsible for the triggering of this severe convective weather. First, the strong thunderstorm high pressure and cold pool formed in Qinhuangdao and Tangshan areas in the northeastern Hebei. The surface wind field was affected by the pressure-gradient force generated by the thunderstorm high pressure, and consequently the northeasterly wind gradually strengthened, which guided the tongue-shaped cold pool to gradually move southwestward to the northern Shijiazhuang. Thus, a long lasting cold-warm boundary (the leading edge of the cold tongue) was formed in the key precipitation area, which provided low-level convergence and the ascending motion for the occurrence and development of this severe convective weather. Second, the northeasterly wind in the east side of Taihang Mountain was forced to ascend by the topography of Taihang Mountain, which strengthened the upward motion. Third, as the direction of the Taihang Mountains shifts from a northeast–southwest direction to a northwest–southeast direction in the west side of Shijiazhuang, the surface northeasterly airflow is changed to a northwesterly airflow here, forming a convergence in the key area of precipitation in front of the mountain. Thus, an unstable vertical structure that features low-level convergence and middle-level divergence is formed. The above three factors work together in the region where the severe convections occurred, and triggered the release of the conditional instability energy, finally resulting in this short-time heavy rain. The results indicate that in the forecast of severe convections in midsummer, we should pay extra attention to the complex terrain and the effects of the cold pool.
... Given the crucial roles of the rear inflows in forming the bowing rainband and the subsequent RSRE process, their origin and development are worthy of further investigations. Previous simulations have shown three different scales of influences in the generation of the storm-relative rear inflows: (i) stronger rear flows near a trough base when the storm moves ahead of the midlevel trough axis slower than the larger-scale mean flow (Bélair & Zhang, 1997;Zhang & Gao, 1989), (ii) rear inflows between a pair of counterrotating mesovortices (Weisman, 1993;Weisman & Davis, 1998;Zhang & Gao, 1989;Zhang, 1992), and (iii) the convectively generated rear-to-front buoyance gradients (Weisman, 1992(Weisman, , 1993. Moreover, using airborne Doppler radar observations, Grim et al. (2009) find that the horizontal pressure gradient generated by the dynamic irrotational wind component is a dominant forcing mechanism at the initial development stage of rear inflows in a rapid developing and decaying squall line. ...
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In this study, an extreme rainfall event of 451 mm in 20 hr that occurred in coastal South China on 11 May 2014 during the Southern China Monsoon Rainfall Experiment is investigated using integrated observations from the dual-Doppler radar pair, polarimetric radar, extensive mesonetwork, and enhanced upper-air soundings. Results show the generation of the extreme rainfall by two consecutive mesoscale convective systems (MCSs) consisting of multiple meso-β-scale rainbands. The two MCSs are maintained by lifting southerly oceanic flows over a quasi-stationary mesoscale outflow boundary (MOB) along the coastline that are enhanced by convectively generated weak cold pool. Northeastward “echo training” of convective cells, under the influence of environmental southwesterly flows, leads to the formation of the multiple rainbands in each MCS. Their subsequent propagations in a “rainband training” form, together with the echo training, along the coastline account for the production of extreme rainfall. The second MCS is characterized with a leading bowing rainband showing a process of rapid splitting and reestablishment (RSRE), which contributes to the formation of the rainband training. The occurrence of the RSRE process requires ample supply of unstable upstream oceanic air mass, the quasi-stationary MOB, and a bowing rainband intersecting with the MOB. The second MCS produces more precipitation than the first one as a result of more rainbands, stronger convective intensity, and more moderate-sized raindrops with larger maximal sizes. The above findings, especially the RSRE process and its associated storm internal circulation, appear to add new Insights into the formation and maintenance of training rainbands and their roles in heavy rainfall production.
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During a hailstorm event, near-surface meso-γ vortices along a convergence line interact with hail cells. Herein we investigate this interaction by using observational data and a high-resolution simulation of a hailstorm that occurred over Taizhou (Zhejiang, China) on 19 March 2014. The 10-m surface wind data from automatic weather stations show that several meso-γ vortices or vortex-like disturbances existed over the convergence zone and played a vital role in the evolution of the hailstorm and the location of the hail. The model results agree with the observations and present a closer correlation between the hail and the low-level meso-γ vortices than those observed. The model simulation indicates that such low-level meso-γ vortices can be used to predict the next 10-min hail fallout zone. The low-level meso-γ vortices originated over the convergence zone and then fed back into the convergence field and provoked a stronger updraft. Vorticity was initiated primarily by stretching and was extended by tilting. A three-dimensional (3-D) flow analysis shows that the existence of low-level meso-γ vortices could help enhance a local updraft. Furthermore, the simulation reveals that the low-level meso-γ vortices formed in the bounded weak echo region (WER) at the front of the hail cell, enhancing convergence and strengthening updrafts. Graupel was broadly located between the 0°C isothermal line and the top of the clouds, roughly between the 0 and −20°C isothermal lines. Accordingly, the hailstones grew rapidly. The suitable environment and the positive effect of the meso-γ vortices on the updrafts enabled hailstorm formation.
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Strong gusty winds in a weak maritime extratropical cyclone (EC) over the Tsushima Strait in the southwestern Sea of Japan capsized several fishing boats on 1 September 2015. A C-band Doppler radar recorded a spiral-shaped reflectivity pattern associated with a convective system and a Doppler velocity pattern of a vortex with a diameter of 30 km [meso- β-scale vortex (MBV)] near the location of the wreck. A high-resolution numerical simulation with horizontal grid interval of 50 m successfully reproduced the spiral-shaped precipitation pattern associated with the MBV and tornado-like strong vortices that had a maximum wind speed exceeding 50 m s ⁻¹ and repeatedly developed in the MBV. The simulated MBV had a strong cyclonic circulation comparable to a mesocyclone in a supercell storm. Unlike mesocyclones associated with a supercell storm, however, its vorticity was largest near the surface and decreased monotonically with increasing height. The strong vorticity of the MBV near the surface originated from a horizontal shear line in the EC. The tornado-like vortices developed in a region of strong horizontal shear in the western part of the MBV, suggesting that they were caused by a shear instability.
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Mid-tropospheric mesoscale convective vortices have been often observed to precede tropical cyclogenesis. Moreover, recent cloud resolving numerical modeling studies that are initialized with a weak cyclonic mid-tropospheric vortex sometimes show a considerable intensification of the mid-level circulation prior to the development of the strong cyclonic surface winds that characterize tropical cyclogenesis. The objective of this two-part study is to determine the processes that lead to the development of a prominent mid-level vortex during a simulation of the transformation of a tropical disturbance into a tropical depression, in particular the role of diabatic heating and cooling. For simplicity simulations are initialized from a quiescent environment. In this first part, results of the numerical simulation are described and the response to stratiform components of the diabatic forcing is investigated. In the second part, the contribution of diabatic heating in convective cells to the development of the mid-level vortex is examined. Results show that after a period of intense convective activity, merging of anvils from numerous cells creates an expansive stratiform ice region in the upper troposphere, and at its base a mid-level inflow starts to develop. Subsequently conservation of angular momentum leads to strengthening of the mid-level circulation. A twelve-hour period of mid-level vortex intensification is examined during which the mid-level tangential winds become stronger than those at the surface. The main method employed to determine the role of diabatic forcing in causing the mid-level inflow is to diagnose it from the full physics simulation and then impose it in a simulation with hydrometeors removed and the microphysics scheme turned off. Removal of hydrometeors is achieved primarily through artificially increasing their fall speeds three hours prior to the twelve-hour period. This results in a state that is in approximate gradient wind balance, with only a weak secondary circulation. Then, estimates of various components of the diabatic forcing are imposed as source terms in the thermodynamic equation in order to examine the circulations that they independently induce. Sublimation cooling at the base of the stratiform ice region is shown to be the main factor responsible for causing the strong mid-level vortex to develop, with smaller contributions from stratiform heating aloft and low level melting and evaporation. This contrasts with the findings of previous studies of mid-latitude vortices that indicate sublimation plays a relatively minor role. An unanticipated result is that the central cool region that develops near the melting level is to a large degree due to compensating adiabatic ascent in response to descent driven by diabatic cooling adjacent to the central region, rather than in situ diabatic cooling. The midlevel inflow estimated from stratiform processes is notably weaker than for the full physics simulation, suggesting a moderate contribution from diabatic forcing in convective cells.
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